1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) exchange system, and more particularly, to an ATM exchange system comprising: a route determination processing unit for receiving multiplexed ATM cells obtained by multiplexing the ATM cells given from each incoming line, and determining each route for the respective received ATM cells; an ATM demultiplexing unit for demultiplexing the output multiplexed ATM cells from the route determination processing unit; and output side buffer units disposed at each outgoing line, for momentarily storing respective ATM cells output from the ATM demultiplexing unit.
The ATM exchange is essential when setting up an ATM communication network, and performs a switching of information of each subscriber terminal by dividing the information into cells. When putting the ATM communication network into practice, various problems that must be solved arise. The present invention is particularly focused on one of these problems, i.e., a technique for performing a switching of a management cell having a high priority and input during a switching of usual ATM cells.
2. Description of the Related Art
In general, two kinds of ATM cells are used; a usual ATM cell to be used by users, in which, the payload composed of general user data is mapped, and an ATM cell which is a management cell and is necessary to the operation and management of the ATM communication network. The payload of the management cell is mapped with important data such as, "SEND", "AIS", information for measuring an absolute delay time, information for notification of a by-pass route and so on. In more detail, the "SEND" data is information used for notifying the subscriber (call origination side) of a service interruption when a signal reception failure occurs at another subscriber (call receiving side); the "AIS" data is information used for notifying a line disconnection; the delay-time measuring data is information used for finding a transmission delay time of the cell, with a high accuracy; and the by-pass route notification data is information used for notifying the other subscriber of the related route when a by-pass route is selected due to traffic congestion or the occurrence of a line fault. Among this management information, if the "SEND" and "AIS" information in particular is not promptly transferred through the ATM exchange, serious damage to the service will occur. Accordingly, management cells containing such information as "SEND", "AIS", and the like are deemed to be management cells having a high priority.
As explained in detail hereinafter, the ATM exchange continuously receives the ATM cells, one by one, from the incoming lines, momentarily stores same therein, and finally, outputs same to the outgoing lines. During the above process, a cell discard may occur, according to a traffic condition of the received cells, and in this case, if the cell discard occurs with respect to the ATM cells containing the user data, the cell discard does not have a serious effect on the overall ATM communication network, per se.
Contrary to the above, if the cell discard occurs with respect to an ATM cell having a high priority, i.e., a management cell important to the system, the cell discard has a very serious effect on the overall ATM communication network per se.
Even if such cell discard does not occur under congested traffic conditions, an adverse transfer delay of the management cells will occur since, in the ATM exchange, the first-in cell becomes the first-out cell and thus a waiting time for each ATM cell is prolonged under such congested traffic conditions.
In consideration of the above, a measure for suppressing cell discards and transfer delays must be used in the ATM exchange system, and such measure has been proposed, although not in the field of the ATM exchange systems but in the field of a packet exchange system for handling packets. Nevertheless, the measure applicable to the packet exchange system cannot be applied to the ATM exchange system because, even though the packets are analogous to the ATM cells, the construction of the system of the packet exchange is very different from that of the ATM exchange.